A variety of rechargeable, high energy density electrochemical cells have been demonstrated although the most widely utilized commercial system is that based upon Li-ion chemistry because it displays very high energy density. Such cells usually include a transition metal oxide or chalcogenide cathode-active material, an anode-active lithium metal or lithium intercalation or alloy compound such as graphitic carbon, tin and silicon, and an electrolytic solution containing a dissolved lithium-based salt in an aprotic organic or inorganic solvent or in a polymer. Today there is great demand for energy storage devices capable of storing more energy per unit volume or per unit mass, e.g., Watt-hours per liter (Wh/l) or Watt-hours per kilogram (Wh/kg), than premier rechargeable Li-ion batteries are capable of delivering. Consequently an increasingly sought after route to meeting this demand higher energy density is to replace the monovalent cation lithium (Li+) with divalent magnesium cations (Mg2+) because magnesium can enable nearly twice the charge of Li+ to be transferred, per volume. Furthermore the abundance of Mg metal and readily available compounds containing Mg is expected to offer significant cost reduction relative to Li-ion batteries. Magnesium also offers superior safety and waste disposal characteristics.
Electrolytes utilizing an alkali metal with organic ligands from organometallic species have been described. Generally the use of an alkaline earth metal anode such as magnesium would appear disadvantageous relative to the use of an alkali metal such as lithium because alkali metal anodes are much more readily ionized than are alkaline earth metal anodes. In addition, on recharge the cell should be capable of re-depositing the anode metal that was dissolved during discharge, in a relatively pure state, and without the formation of deposits that block the electrodes. One practiced in the art would note this characteristic is not natural for Mg. Despite this, there are numerous other disadvantages to alkali batteries. Alkali metals, and lithium in particular, are expensive and highly reactive. Alkali metals are also highly flammable, and fire caused by the reaction of alkali metals with oxygen, water or other reactive materials is extremely difficult to extinguish. As a result, the use of alkali metals requires specialized facilities, such as dry rooms, specialized equipment and specialized procedures, and shipment of Lithium containing products (e.g., batteries) is tightly controlled. In contrast, magnesium metal and its respective inorganic salts are easy to process and usually are considered as benign. Magnesium metal is reactive, but it undergoes rapid passivation of the surface, such that the metal and its alloys are highly stable. Magnesium is inexpensive relative to the alkali metals, and widely used as ubiquitous construction materials.
Known electrolytes that enable reversible, electrochemical deposition of Mg and that have potential use in a battery contain organometallic materials. Most often these electrolytes contain organometallic Grignard salts as the electrochemically active component. However sustaining anodic limits greater than 1 Volt is problematic or impossible with the usual intercalation cathodes because of electrolyte decomposition and corresponding encrustation and/or passivation of electrode surfaces. The anodic limit, or anodic voltage, is a measure of an electrolytes stability limit; represented as the highest voltage that can be applied to the electrolyte prior to initiating oxidative decomposition of the electrolyte at an electrode surface. Enhanced electrochemical stability has been demonstrated by complexing Grignard reagents with strong Lewis acids. For example, a cell comprised of a magnesium metal anode, a molybdenum sulfide “Chevrel” phase active material cathode, and an electrolyte solution derived from an organometallic complex containing Mg is capable of the reversible, electrochemical plating of magnesium metal from solutions with about a 2 V anodic limit of the stability window. Under the same principle similar results have also been shown when Magnesium Chloride and organometallic Aluminum compounds complexes are employed.
Such cells are low energy density due to a low difference in operating potentials between a Chevrel cathode and Mg metal anode and therefore are not commercially viable cells. Sustaining an anodic voltage greater than 2 volts is problematic or impossible with the usual intercalation cathodes and electrolytes based upon Grignard reagents and other organometallic species. Magnesium batteries operating at voltages greater than 1.5 volts are particularly prone to electrolyte decomposition and to encrustation and/or passivation of the electrode surface due to anodic limits of the electrolyte. Furthermore electrolytes intended for use in electrochemical cells in which the plating and stripping of Mg ions is required include organometallic species among the ionic species in the respective electrolytic solutions. There are many disadvantages to organometallic species, relative to inorganic salts. Practically, all organometallic species of the alkalis and the earth alkalis are highly unstable in the presence of air and water and thus are classified as pyrophoric. Organometallic species of sufficient purity are quite expensive to produce. Organometallic species introduce organic ligands into the electrolytic solution, which will limit the chemical stability of the solution when in contact with certain electrode active materials and other electrochemical cell components. In general, handling, manipulation and storing organometallic species of this sort are complicated, hazardous and expensive.
In contrast one practiced in the art will recognize that previous attempts to utilize inorganic magnesium salts failed to enable substantial reversibility of magnesium deposition with high Coulombic efficiency and low overpotential. In general it has been shown that electrodeposition in previous inorganic magnesium salt solutions corresponded with electrolyte consumption and resulted in decomposition of the solution components. The decomposition products passivate the electrode blocking in further electrochemical reaction. Consequently no commercial Mg secondary batteries have succeeded thus far.
The literature on Mg secondary batteries includes N. Amir et al., “Progress in nonaqueous magnesium electrochemistry,” Journal of Power Sources 174 (2007) 1234-1240, published on line on Jun. 30, 2007; Y Gofer et al., “Magnesium Batteries (Secondary and Primary),” published in Encyclopedia of Electrochemical Power Sources 2009 285-301 Elsevier B. V.; and John Muldoon et al., “Electrolyte roadblocks to a magnesium rechargeable battery,” 5 (2012) Energy & Environmental Science 5941-5950.
Also previously described is Aurbach et al. in U.S. Pat. No. 6,316,141, issued Nov. 13, 2001, which is said to disclose a cell comprised of a Magnesium metal anode, a Molybdenum Sulfide “Chevrel” phase active material cathode, and an electrolyte solution derived from an organometallic complex containing Mg. The critical aspect of that invention is the specification of an electrolyte capable of the reversible, electrochemical plating of Magnesium metal from solutions with a 2 V anodic limit. This was demonstrated through the formation of complex electrolytically active salts represented by the formula: M′+m(ZRnXq-n)m in which: M′ is selected from a group consisting of magnesium, calcium, aluminum, lithium and sodium; Z is selected from a group consisting of aluminum, boron, phosphorus, antimony and arsenic; R represents radicals selected from the following groups: alkyl, alkenyl, aryl, phenyl, benzyl, and amido; X is a halogen (I, Br, Cl, F); m=1-3; and n=0-5 and q=6 in the case of Z=phosphorus, antimony and arsenic, and n=0-3 and q=4 in the case of Z=aluminum and boron.
In a different report Nakayama et. al., U.S. Patent Application Publication No. 2010/0136439, published Jun. 3, 2010, which is said to disclose a magnesium ion-containing nonaqueous electrolytic solution comprising a magnesium ion and another kind of a metal ion dissolved in an organic solvent, wherein solutions may be obtained through combinations of inorganic Lewis Base MgCl2 and organometallic Aluminum Lewis Acids such as dimethylaluminum chloride or methylaluminum dichloride.
Also described is Yamamoto et al., U.S. Patent Application Publication No. 2009/0068568, published Mar. 12, 2009, which is said to disclose a magnesium ion containing non-aqueous electrolyte in which magnesium ions and aluminum ions are dissolved in an organic ethereal solvent, and which is formed by adding metal magnesium, a halogenated hydrocarbon, an aluminum halide AlY3, and a quaternary ammonium salt to an organic ethereal solvent and applying a heating treatment while stirring them as a one-step reaction to form the Grignard-based organometallic containing complex solution species.
There is a need for improved non-aqueous electrolytes for use in secondary batteries.